1. Field of the Invention
The present invention relates to a method and apparatus for determining the density of microorganisms such as bacteria and fungi floating in the air (hereinafter referred to as "air borne bacteria").
2. Description of the Prior Art
Determination of the density (number per unit volume) of air borne bacteria is performed as one of the means to control the cleanliness in a room, to determine environmental pollution and like purposes. Falling bacteria method and collision method have been used for determining the density of air borne bacteria.
The former comprises allowing an open schale containing an agar medium to stand on a floor for a designate period to collect and incubate the bacteria falling from the air during the period and then measuring the number of colonies developed on the medium. This method, while having the advantage of ready measurement procedure, has problems of being incapable of conducting quantitative determination of the density of bacteria and of poor reproducibility caused by large variation in the values measured. The latter method comprises sucking a specified volume of air from the top of a measuring apparatus, and collide the sucked air onto an agar medium to collect bacteria, for which various measuring apparatuses are available. Slit-sampler type measuring apparatus is generally used for this purpose. This apparatus has on its top an air suction port comprising a narrow slit, through which air is blown onto a schale placed on a turn table mounted below the slit. The method, while being capable of conducting quantitative determination, has problems of the apparatus being expensive and the bacteria collected being readily affected by drying up because of high air flow rate.
In recent years, what is known as filtration method, utilizing a membrane filter, has been attracting much attention instead of the above falling bacteria and collision methods. This method comprises passing a specified volume of air intermittently through a membrane filter housed in a holder, thereby collecting bacteria on the surface of the membrane filter. The filter is then taken out from the holder and the bacteria collected on the filter surface is washed out into a liquid medium or the filter with the bacteria is laid on an agar medium to incubate the bacteria, followed by measurement of the number of the colonies developed on the surface of the membrane filter.
This filtration method has higher reproducibility and is better than falling bacteria method and collision method. The method however cannot be said to have high accuracy with sufficient reproducibility, since, when for example air borne bacteria in the same room are analyzed, the measurement data tend to vary to a large dispersion because of the bacteria dying out depending on the measuring conditions, e.g. rate and direction of air suction. Furthermore both the above collision and filtration methods require time-consuming sterilization and pasteurization of the apparatus used, preparation of medium and like preparations, and can be conducted only by skilled operators.
The large dispersion of measurement data in the filtration method is attributable firstly to the fact that, when air borne bacteria are collected through a membrane filter, the bacteria damage their cell membrane by collision against the membrane filter or die out due to intermittent application of pressures. It is therefore most important, for the purpose of increasing the reproducibility, to collect bacteria as they are, without killing them.
The large dispersion is secondly attributable to contamination of other bacteria occurring when the bacteria collected on the membrane filter is washed out on a liquid medium for incubation or the membrane filter with the bacteria is placed upon a medium, after the membrane filter with the bacteria has been removed from the holder.